Acquisition system and method for towed electromagnetic sensor cable and source

ABSTRACT

An electromagnetic survey acquisition system includes a sensor cable and a source cable, each deployable in a body of water, and a recording system. The sensor cable includes an electromagnetic sensor thereon. The source cable includes an electromagnetic antenna thereon. The recording system includes a source current generator, a current sensor, and an acquisition controller. The source current generator powers the source cable to emit an electromagnetic field from the antenna. The current sensor is coupled to the source current generator. The acquisition controller interrogates the electromagnetic sensor and the current sensor at selected times in a synchronized fashion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/467,261 filed May 9, 2012 (PGS-10-32US) titled, “Acquisition Systemand Method for Towed Electromagnetic Sensor Cable and Source”, which isincorporated by reference herein as if reproduced in full below.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

The invention relates generally to the field of marine electromagneticsurveying.

More particularly, the invention relates to towed streamer-type marineelectromagnetic surveying and acquisition systems having reduced signalsensitivity to vessel and streamer motion in a body of water.

Marine electromagnetic surveying includes acquisition of electromagneticsignals from formations below the bottom of a body of water usingelectromagnetic sensor streamers that may be towed by a vessel in thebody of water. An electromagnetic energy source may also be towed by thesame vessel or by a different vessel.

U.S. Pat. No. 7,671,598 issued to Ronaess et al. describes a system andmethod for reducing induction noise in a marine electromagnetic surveysystem resulting from motion of the various sensor streamer componentsin the water. High quality electromagnetic data acquisition using towedstreamer(s) and a towed electromagnetic energy source may requiredetermining noise that may be induced in other components of theacquisition system, such as the tow vessel and/or the electromagneticenergy source as they move along the body of water.

Thus there exists a need for a marine electromagnetic survey system andmethod that can provide reduced vessel and source motion-induced noisein the acquired signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example embodiment of an acquisition system for marineelectromagnetic surveying.

FIG. 2 shows an example embodiment of electromagnetic signal generationand recording devices that may form part of the recording systemexplained with reference to FIG. 1.

FIG. 3 shows another example embodiment of electromagnetic signalgeneration and recording devices.

FIG. 4 shows another example embodiment of electromagnetic signalgeneration and recording devices.

FIG. 5 shows an example embodiment of a sensor cable.

FIG. 6 shows an example embodiment of a signal processing andconfiguration module

DETAILED DESCRIPTION

An example embodiment of an acquisition system for marineelectromagnetic surveying is shown schematically in FIG. 1. A surveyvessel 10 moves along the surface of a body of water 11 such as a lakeor an ocean. The survey vessel 10 may include equipment, shown generallyat 12 and referred to for convenience as a “recording system” that maycomprise devices (none shown separately in FIG. 1) for applying electriccurrent to an antenna such as source electrodes 18 and/or other devicesdisposed on or along a source cable 14, which may be towed by the surveyvessel 10. The recording system 12 may also include equipment (not shownseparately in FIG. 1) for navigating the survey vessel 10, fordetermining the geodetic position of the survey vessel 10 and ofcomponents towed by the survey vessel 10 in the body of water 11, anddevices for recording signals detected by one or more sensors on one ormore sensor cables 16. As shown in FIG. 1, the sensor cable(s) 16 mayalso be towed by the survey vessel 10. In other embodiments, sensorcable(s) 16 may be towed by another survey vessel (not shown).

The source cable 14 in the present example may include anelectromagnetic antenna consisting of two source electrodes 18 disposedat spaced apart positions along the source cable 14. At selected timescertain of the equipment (not shown separately in FIG. 1) in therecording system 12 applies electric current across the sourceelectrodes 18. The time varying components of such electric currentproduce an electromagnetic field that propagates through the body water11 and into the formations 30 below the water bottom 19. The particulartype of current conducted across the source electrodes 18 may be variousforms of switched direct current, such used in transient controlledsource electromagnetic surveying or types of current used in frequencydomain electromagnetic surveying. Non-limiting examples of switcheddirect current for transient controlled source electromagnetic surveyinginclude switching the current on, switching the current off, reversingcurrent polarity and selected switching sequences such as pseudo-randombinary sequences. It is within the scope of the invention, therefore, toperform either or both frequency domain and transient controlled sourceelectromagnetic surveying. It should also be understood that thearrangement of the source electrodes 18 shown in FIG. 1, referred to asa horizontal electric dipole antenna, is not the only type ofelectromagnetic source antenna that may be used with the invention. Thesource cable 14 may also include, in addition to or in substitution ofthe horizontal electric dipole source antenna shown in FIG. 1, any oneor more of a vertical electric dipole antenna, and horizontal orvertical magnetic dipole antenna (current loop). Accordingly, theelectromagnetic field source antenna configuration shown in FIG. 1 isnot intended to limit the scope of the present invention.

In the example embodiment of FIG. 1, the survey vessel 10 may also towmore than one sensor cable 16. A single sensor cable is shown in FIG. 1only for clarity of the illustration and is not a limit on the scope ofthe invention. The sensor cable 16 may include at least oneelectromagnetic sensor 20, and preferably a plurality of suchelectromagnetic sensors disposed at spaced apart positions along thelength of the sensor cable 16. Each of the one or more electromagneticsensors 20 may measure a parameter related to the electromagnetic fieldresulting from interaction of the electromagnetic field induced by thesource (e.g., source electrodes 18) with the subsurface formations 30below the water bottom 19. In the present example, the electromagneticsensors 20 may be pairs of receiver electrodes disposed at spaced apartpositions along the sensor cable 16. An electric field component of theelectromagnetic field resulting from interaction of the inducedelectromagnetic field with the formations 30 below the water bottom 19can induce voltages across each of the pairs of receiver electrodes, andsuch voltages may be detected by any form of voltage measuring circuit(not shown in the present figure) known in the art. Such voltagemeasuring circuits (not shown) may be disposed in the sensor cable 16and/or in the recording system 12.

Another example of an electromagnetic sensor that may be used in otherexamples is a single axis or multi-axis magnetometer, such as a fluxgate magnetometer.

It should be understood that the example electromagnetic survey systemof FIG. 1 including only one sensor cable 16 is shown to illustrate howto make and use a sensor cable according to various aspects of theinvention. A sensor cable according to the various aspects of theinvention may be used with acquisition systems that include a pluralityof laterally spaced apart sensors cables towed by the survey vessel 10and/or by another vessel in a selected configuration to provide “inline” and “cross line” electromagnetic and/or seismic signals.Accordingly, the number of sensor cables and their particular geometricconfiguration in the water 11 are not limits on the scope of the presentinvention.

If electrode pairs are used as the electromagnetic sensors 20, suchelectrode pairs may measure voltages induced by the electromagneticfield generated as a result of the interaction of the inducedelectromagnetic field with the formations 30 below the water bottom 19.It will be appreciated by those skilled in the art that motion of thesurvey vessel 10 and motion of the sensor cable(s) 16 through the water11 may not be uniform. Such non-uniform motion may result from currentsin the water and acceleration of the survey vessel 10 (change invelocity or direction) transferred to the sensor cable 16 through towingequipment used to connect the sensor cable 16 to the survey vessel 10.Such non-uniform motion of the sensor cable 16 through the Earth'smagnetic field can induce voltages along electrical conductors (notshown) in the sensor cable 16 as well as in the electromagnetic sensors20. The motion-induced voltages may be calculated or estimated if themotion of sensor cable 16 proximate the electromagnetic sensors 20 isknown. In the present example, motion sensors 25 may be disposed atselected positions along the sensor cable 16. In the example of FIG. 1,the motion sensors 25 are each shown as located proximate to one of theelectromagnetic sensors 20. The example number of motion sensors 25 andtheir placement as shown in FIG. 1 are not intended to limit the numberof motion sensors or their particular geometric configuration that maybe used in other examples of an electromagnetic sensor cable accordingto the invention. The signals measured by the motion sensors 25 may bedetected and processed by certain equipment (not shown in FIG. 1) in therecording system 12, and may be used to estimate magnitude of inducedvoltages resulting from motion of parts of the sensor cable 16 withrespect to the Earth's magnetic field. Such estimates may be used inprocessing measurements made from the electromagnetic sensors 20 in thesensor cable 16 to reduce the effects of the motion induced voltages onthe measurements made by the electromagnetic sensors 20. An exampleembodiment for using the signals detected by the motion sensors 25 tocorrect measurements made by the electromagnetic sensors 20 is describedin U.S. Pat. No. 7,671,598 issued to Ronaess et al. and incorporatedherein by reference.

In some embodiments, similar motion sensors may be included on board thesurvey vessel 10, for example, in or proximate the recording system 12.A schematic diagram showing an example embodiment of an acquisitionsystem including such additional sensors may be better understood withreference to FIG. 2. The term “acquisition system” as used herein isintended to mean all the components typically used to generate, detectand record electromagnetic survey signals. The acquisition system thusmay include the recording system 12, the source cable 14 and at leastone sensor cable 16.

The recording system 12 may include a source current generator 48 thatmay provide one or more types of alternating current or switched directcurrent as previously explained. A “signature”, or amplitude withrespect to time of the current provided by the source current generator48 may be measured by one or more current sensors 40, 42 coupled to anoutput of the source current generator 48. The current sensors 40, 42may generate an electrical and/or optical signal corresponding to thecurrent magnitude with respect to time. The recording system 12 may alsoinclude a data acquisition controller 50, which may be, for example, abuilt-to-purpose microcomputer or suitably programmed general purposemicrocomputer. The acquisition controller 50 may obtain an absolute timereference signal from, for example, a geodetic position signal receiver52, such as a GNSS signal receiver. The absolute time reference signalmay be used to synchronize signal detection and recording functions, sothat signal amplitude of the output of all the sensors in theacquisition system may be recorded with respect to an identical timereference.

The acquisition controller 50 may send an interrogation command signalover a command signal line 54, which may be, for example, one or moreinsulated electrical conductors, or one or more optical fibers. Theinterrogation command signal may interact with each of a plurality ofacquisition nodes 60 in the sensor cable 16. The acquisition nodes 60may include circuitry, for example, for detection of voltages impressedacross pairs of electrodes or other electromagnetic sensor(s) (e.g., 20in FIG. 1) and for detection of signals from the motion sensors (25 inFIG. 1). The signals detected by each sensor may be converted bycircuits (not shown separately) associated with each acquisition node 60into a form suitable for transmission to the recording system 12 over asignal return line 55 (FIG. 3). Example circuitry that may be used toconvert the detected voltages and motion signals into suitable form forcommunication, such as optical signals, is described in the Ronaess etal. '598 patent. Conversion of the foregoing detected signals intoelectrical telemetry signals is also within the scope of the presentinvention. In the example embodiment in FIG. 2, the command signal line54 may extend the entire length of the sensor cable 16 and may return tothe survey vessel (10 in FIG. 1) and to acquisition controller 50 usingthe signal return line 56. The signal return line 56 may extend to theone or more sensors disposed on the survey vessel (10 in FIG. 1) such asthe current sensors 40, 42, and motion sensors 44, 46. The motionsensors 44, 46 may be similar in configuration to the motion sensors (25in FIG. 1) in the sensor cable 16. The signals emitted by each of the onboard current sensors 40, 42, and motion sensors 44, 46 may be convertedto a form, such as optical signals, suitable for transmission to theacquisition controller 50 using the signal return line 56. Signals fromthe motion sensors 44, 46 on board the survey vessel (10 in FIG. 1) maybe used substantially as explained in the Ronaess et al. '598 patent tofurther reduce motion induced noise in the signals generated by theelectromagnetic sensors (20 in FIG. 1) in the sensor cable (16 in FIG.1).

The acquisition controller 50 may also generate control signals foroperation of the source current generator 48. Current from the sourcecurrent generator 48 may be conducted to the source electrodes 18 in thesource cable 14. Signals detected by the various sensors may thereby beprecisely synchronized. For example, by having the acquisitioncontroller 50 operate both signal generation through control of thesource current generator 48 and detection of signals from the varioussensors and acquisition nodes by sending an interrogation signal to eachacquisition node 60 and on board current sensors 40, 42, the sourcecurrent may be synchronized with signals from the acquisition nodes.Acquisition controller 50 may also detect signals from motion sensors44, 46, allowing the signals detected by the motion sensors to beprecisely synchronized with the source current. In other exampleembodiments, the acquisition controller 50 may only provide controlsignals to synchronously interrogate the on board current sensors 40,42, and motion sensors 44, 46 and each acquisition node 60.

The example embodiment shown in FIG. 2 provides that all the sensors inthe acquisition system may be interrogated using a single command signalline 54 with an associated signal return line 56 in communication withall the sensors and acquisition nodes 60. In another example embodiment,shown in FIG. 3, a separate command signal line 54 and signal returnline 55 may be used to obtain signals from the sensor cable 16. The onboard current sensors 40, 42, and motion sensors 44, 46 may haveseparate provision for signal communication with the acquisitioncontroller 50. Because acquisition of all signals may be performed bythe same acquisition controller 50, the acquisition may be synchronizedas accurately or more accurately as in the example embodiment shown inFIG. 2. Other components of the acquisition system shown in FIG. 3 maybe substantially the same as those of the example embodiment of theacquisition system shown in FIG. 2.

A particular embodiment of equipment in the recording system 12 that mayprovide reduced noise in the detected signals from each of the sensorsas well as reduced cross-coupling between sensor signals and between thesenor signals and the current imparted to the source cable (14 inFIG. 1) may be better understood with reference to FIG. 4. Theacquisition controller 50 may be operated from its own power supply. Insome embodiments, the power supply for acquisition controller 50 mayconsist of one or more storage batteries 50C, rechargeable by a batterycharger 50D, a DC to AC converter 50B coupled to the storage batteries50C and a power unit 50A coupled to the output of the DC to AC converter50B. The power unit 50A may provide electric power to operate theacquisition controller 50 and the circuits (not shown) in the sensorcable (16 in FIG. 1). Acquisition command signals from the acquisitioncontroller 50 may be conducted in electrical form to an optical signaltransceiver 57. The optical signal transceiver 57 may include its ownseparate power supply 53 so that any variations in the power used tooperate the acquisition controller 50 will not affect the signalsgenerated by the optical signal transceiver 57. As explained withreference to FIG. 2, command signals from the acquisition controller 50may be conducted to the sensor cable (16 in FIG. 1) using a commandsignal line 54. In the present embodiment, the command signal line maybe one or more optical fibers. Signals returned from the variousacquisition nodes (60 in FIG. 2) and on board current sensors (e.g., 40,42 in FIG. 2) may be conducted to the acquisition controller 50 using asignal return line 56.

It will be appreciated by those skilled in the art that the sensor cable(16 in FIG. 1) may be deployed into the water using a winch or similarspooling device. Typically a winch or similar spooling device thatextends and retracts cables having insulated electrical conductorsand/or optical fibers will include a slip ring set 17A or similar deviceenabling relative rotation between the winch reel and rotationally fixedconnections to the electrical conductors and/or optical fibers on thecable. In the present embodiment, the sensor cable (16 in FIG. 1) mayuse optical fibers, e.g., command signal line 54 and signal return line56, for communication of command signals and sensor signals, and may useinsulated electrical conductors 20A to transmit electrical power to thecircuits (not show separately in FIG. 4) in the various components ofthe sensor cable (16 in FIG. 2). In the present embodiment, the winchmay include optical to electrical converters 17B on both the rotatingpart of winch and on the rotationally fixed part, and in signalcommunication with the command signal line 54 and the signal return line56. The optical to electrical converters 17B may enable signals to betransmitted in electrical form through the slip rings 17A, while beingtransmitted in optical form along the sensor cable (16 in FIG. 2) and inthe recording system 12. Other embodiments may use optical slip ringsfor transmission of optical signals between the rotating and fixed partsof the winch

In the present example embodiment, the source current generator 48 andassociated current sensors 40, 42 may include their own separate powersupply 48A. Such separate power supply 48A may be used to isolate thelarge current generated by the source current generator 48, so that thelarge current from the source current generator 48 may only minimallyaffect the other components of the recording system 12. Output of thesource current generator 48 may be applied to the source cable 14 asexplained with reference to FIG. 1.

An example of a sensor cable 16 is shown schematically in FIG. 5. Thesensor cable 16 may include a lead in 17 or similar device to enabletowing by the survey vessel (10 in FIG. 1). The sensor cable 16 may beassembled from sensor segments 16A coupled end to end in pairs. Eachpair of sensor segments 16A may be coupled to a succeeding pair ofsensor segments 16A through a signal processing and configuration module16B. The modules 16B may each contain one or more of the acquisitionnodes (60 in FIG. 3). The sensor segments 16A may include suitableelectromagnetic sensors 20 as explained with reference to FIG. 1. Themotion sensors are omitted from FIG. 5 for clarity of the illustration.

An example embodiment of one of the signal processing and configurationmodules 16B (hereinafter “module”) is shown in cut away view in FIG. 6.The module 16B may be enclosed in a pressure resistant housing 131 suchas may be made from high strength plastic or non-magnetic steel alloy.The housing 131 may be substantially cylindrically shaped, and mayinclude electrical/mechanical terminations 130 configured to couple tothe terminations 130B on a sensor segment 16A. The example embodimentshown in FIG. 6 may provide for such connection between cable segmentsby including a flange 134 on the exterior of the housing 131 whichengages a mating flange (not shown) in a connecting sleeve 133. Theconnecting sleeve 133 may be substantially cylindrical in shape, andwhen moved along the exterior of the housing 131 may engage o-rings 135or similar seal elements positioned longitudinally on either side of anopening 132 in the wall of the housing 131. Thus, with the sleeve 133removed, the opening 132 is accessible. With the sleeve 133 in theconnected position, for example, by engaging internal threads 133A onthe end of the sleeve 133 with mating threads 130A on the adjacenttermination 130B of sensor segment 16A, the interior of the housing 131is sealed from water intrusion by the sleeve 133.

The interior of the housing 131 may include circuits for selectiveelectrical interconnection of the various sensor segments 16A andconnection of sensors on the sensor segments 16A to voltage or currentmeasuring circuitry. In the present embodiment, each of the electricalconnections 146 at each end of the housing 130 may be electricallyconnected, such as by twisted pairs of wires to a correspondingelectrical contact on a configuration plug receptacle 136. Otherelectrical contacts on the configuration plug receptacle 136 may connectto the input terminals of one or more low noise amplifier/digitizer(LNA/ADC) combinations 137. Output of the combination(s) 137 may becoupled to an electrical to optical signal converter (EOC) 138 andthence to the one or more optical fibers 54 for communication ofdigitized voltage signals along the sensor cable and if required to therecording system (12 in FIG. 1). Power for the LNA/ADC 137 and EOC 138may be provided by a battery (not shown) inside the module 16B. Suchbattery may be rechargeable while the sensor cable is deployed by usinga module charging circuit such as one described in U.S. Pat. No.7,602,191 issued to Davidsson, incorporated herein by reference.

The particular sensors on any sensor segment 16A located either towardthe forward or aft end of the sensor cable (16 in FIG. 5) with respectto the module 16B that are connected to through wires or to the input ofthe LNA/ADC combination 137 may be selected prior to deployment of thesensor cable by inserting a suitably wired configuration plug 136A intothe receptacle 136. Thus, in combination, suitable numbers of sensorsegments 16A, and suitably configured modules 16B may provide the systemuser with a large number of options as to electrode spacing and offsetwhile making only two basic sensor cable components.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method for electromagnetic surveying,comprising: inducing an electromagnetic field in a body of water with asource current; detecting a property of the source current at selectedtimes with a current sensor; detecting a property of an electromagneticfield in the body of water at selected time with an electromagneticsensor; and synchronizing the detecting the property of the sourcecurrent with the detecting the property of the electromagnetic field. 2.The method of claim 1, further comprising synchronizing the detectingthe property of the source current and the detecting the property of theelectromagnetic field with an absolute time reference.
 3. The method ofclaim 1, further comprising: measuring motion of the electromagneticsensor; measuring motion of a survey vessel including thereon arecording system for recording the detected property; and synchronizingthe detecting the property of the electromagnetic field, measuringmotion of the electromagnetic sensor, and measuring motion of the surveyvessel.
 4. The method of claim 3 wherein the inducing theelectromagnetic field is synchronized with the detecting the property ofthe electromagnetic field, measuring motion of the electromagneticsensor, and measuring motion of the survey vessel with an absolute timereference signal.
 5. The method of claim 1 further comprising using thedetected property of the source current and the detected property of theelectromagnetic field to determine a response of subsurface formationspresent in the electromagnetic field.
 6. The method of claim 3 furthercomprising determining a motion induced component of the detectedproperty of the electromagnetic field using the measured motion of theelectromagnetic sensor and measured motion of the survey vessel.
 7. Themethod of claim 1 wherein the detected property of the electromagneticfield comprises at least one of electric field gradient and electricpotential.
 8. The method of claim 2 wherein the absolute time referencesignal is obtained by detecting a geodetic position signal.
 9. Themethod of claim 1 wherein the induced electromagnetic field comprises atleast one of an alternating electromagnetic field and a transientelectromagnetic field.
 10. The method of claim 1 further comprisingactuating the first source current generator in response to selectedsignals from the electromagnetic sensor.